The first is that it gets your brand in front of people. Even if you do not find your next all-star employee, you are representing your company as an established, stable place to work.
The second benefit to job fairs is a more selfish one. You know that pushy job-seeker who shoved their resume in your face and made no effort to connect with you? You saw how ineffective that was at impressing you, so if you ever find yourself on the job hunt again, you will know what type of behavior at job fairs makes a good impression on employers.
Career fairs may not be your favorite way to spend a day, but appreciate what they can do for you and your business!